1. FIELD OF THE INVENTION
The invention relates to nanoporous dielectric films and to a process for their manufacture. Such films are useful in the production of integrated circuits.
2. DESCRIPTION OF THE PRIOR ART
The desire for lower dielectric constant materials for use as intermetal and interlevel dielectrics for technology nodes at 0.18 micron feature sizes and below is well-known in the semiconductor industry. Although the need for obtaining such films has been known for a number of years, commercially available materials have been limited to a those having a dielectric constant in the range of k&gt;2.7. Although low dielectric constant materials are required, a number of other criteria must also be met for a useful dielectric material in the semiconductor uses. These criteria include electrical leakage, breakdown, and electromigration; high chemical purity, a six month or better storage life, low moisture adsorption, chemical resistance; thickness uniformity, low stress, low shrinkage, and crack resistance; high thermal stability, low thermal expansion, low thermal weight loss; and low cost. Nanoporous silicas are good materials which meet these criteria and also offer dielectric constants of 2.7 and below.
In addition to a low dielectric constant, nanoporous silica offers other advantages for microelectronics applications including thermal stability up to 900.degree. C., pore sizes which are smaller than microelectronics features; a material that is widely used in the semiconductor industry, namely silica and its precursors; deposition using tools simiar to those employed for conventional spin-on glass (SOG) processing; as well as the ability to tune dielectric constants over a wide range k=1.3-2.5. Nanoporous silicas also avoid thickness constraints from cracking as observed with conventional SOG's, and have the ability to migrate the same dielectric material and integration scheme for multiple semiconductor technology nodes by tuning the dielectric constant to lower values. Although nanoporous silica has these advantages, it also may suffer from several disadvantages which are common to most SOG materials. These include a relatively large raw material consumption. For a 200 mm wafer, 3 to 8 cm.sup.3 of silica precursor is typically deposited for each dielectric layer. However, the actual volume of the film is on the order of 0.1 cm.sup.3. Therefore, a significant fraction of the silica precursor is lost which results in higher costs to the IC manufacturer. A large volume of solvent is typically used in SOG and nanoporous silica precursors in order to lower viscosity for deposition. However, the evaporation of the solvent results in a concentration of impurities in the film. As IC dimensions continue to shrink, IC manufacturers require ever lower impurity levels which thus require the use of extremely pure solvents which adds a nontrivial expense to the precursor production. As a result of changes in fluid dynamics and mass transfer across the wafer, the problem of achieving film uniformity, thickness and refractive index can be difficult. This problem becomes more difficult as substrate sizes increase and for nonuniform shapes such as flat panel displays. Often, spin-on materials suffer from the appearance of a number of different kinds of film defects arising from the complex drying and polymerization processes. Furthermore, the extent of local and global planarization depends on a complex interrelationships among a number of variables. Using a deposition technique other than spin deposition could result in different planarization results.
This invention solves the above problems by depositing a nanoporous silica precursor on a wafer by condensation of silica presursors from the vapor phase. In this way, essentially all of the precursor is transformed into silica, resulting in much higher yields, lower solvent consumption, and higher purity. In addition, the film uniformity is better than for films deposited by a liquid spin-on glass technique.
According to the invention, a silica precursor is deposited onto the wafer from a vapor. This may be conducted by silica precursor deposition from the vapor phase to form a liquid-like film on the wafer surface. This may also include co-deposition from the vapor phase of a solvent, and/or solvent vapor deposition before or after the silica precursor. Polymerization and gelation caused by exposure of the silica precursor to an initiator or catalyst such as an acidic or basic vapor, water vapor, thermal means, light or other means that cause gelation. This intermediate product is a wet gel film in which the pores of the film contain a fluid which can be removed by subsequent drying. Drying the polymerized film then yields a porous silica film with pore size on the order of nanometers. Additional optional steps may include a treatment to make the film hydrophobic, a heat treatment before polymerization to aid in planarization and gap filling, i.e. reflow, and/or aging and thermal curing before or after drying to increase film strength.